1. Field of the Invention
This invention relates to the blending together of polytetrafluoroethylene and melt-fabricable perfluoropolymer.
2. Description of Related Art
US 2004/0242783 A1 discloses a blend of tetrafluoroethylene/hexafluoropropylene copolymer, commonly called FEP, and polytetrafluoroethylene (PTFE), the PTFE imparting the improved extrusion property of reduced cone breaks during melt draw down extrusion coating of wire. The FEP composition by itself is selected to provide good crack resistance for molded articles, and the PTFE has a high enough molecular weight so that the molded article does not have poor crack resistance. The PTFE content of the blend is disclosed to be 0.03 to 2 parts by weight based on 100 parts by weight of the copolymer. When the amount of PTFE is more than 2 parts by weight, two disadvantageous results are disclosed: the melt viscosity of the blend increases significantly and the molded article tends to become brittle
The FEP and PTFE are blended together by melt kneading. Before kneading, the copolymer and PTFE can be pre-mixed to improve the degree of dispersion of the PTFE [0042]. In Example 1, powders of the PTFE and copolymer are mixed together, followed by kneading in a twin-screw extruder to produce molding pellets, which are then melt-extruded, using a single screw extruder, as a coating onto wire. The PTFE powder has an average particle size of 450 micrometers. The particle size of the copolymer is not disclosed in Example 1, but the aqueous emulsion polymerization to obtain this copolymer is disclosed. The copolymer is recovered from emulsion polymerization by coagulation, which provides a dry powder particles having an average size about the same as the PTFE powder.